1. Field of the Invention
This invention relates generally to a method for determining whether a fuel cell stack is overheating and, more particularly, to a method for determining whether a fuel cell stack is overheating using end cell temperature sensors.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
For automotive applications, it typically takes about 400 fuel cells to provide the desired power. Because so many fuel cells are required for the stack in automotive fuel cell system designs, the stack is sometimes split into two sub-stacks each including about 200 fuel cells because it is difficult to effectively provide an equal flow of hydrogen gas through 400 fuel cells in parallel.
A fuel cell system typically includes a thermal sub-system for cooling the fuel cell stack to a desired operating temperature. The thermal sub-system includes a pump that pumps a cooling fluid through a coolant loop outside of the stack and cooling fluid flow channels provided within the bipolar plates. A radiator typically cools the hot cooling fluid that exits the stack before it is sent back to the stack.
The end cells in a fuel cell stack typically have a lower performance than the other cells in the stack. Particularly, the end cells are exposed to ambient temperature, and thus have a temperature gradient that causes them to operate at a lower temperature as a result of convective heat losses. Because the end cells are typically cooler than the rest of the cells in the stack, gaseous water more easily condenses into liquid water so that the end cells have a higher relative humidity, which causes water droplets to more readily form in the flow channels of the end cells. Further, at low stack load, the amount of reactant gas flow available to push the water out of the flow channels is significantly reduced. Also, at low stack loads the temperature of the cooling fluid is reduced, which reduces the temperature of the stack and typically increases the relative humidity of the reactant gas flow.
It is known in the art to heat the end cells with resistive heaters positioned between a unipolar plate and an MEA so as to compensate for convective heat losses. These systems typically attempted to maintain the end cell temperature the same as the other cells in the stack by monitoring the temperature of the cooling fluid out of the stack. However, lower cell voltages for the end cells may still be a problem even with the addition of such heaters.
Various components in the fuel cell stack, such as the membranes, may be damaged if the temperature of the stack increases above a certain materials transition temperature, such as 85° C. Therefore, fuel cell systems typically employ a cooling fluid temperature monitoring sub-system that monitors the temperature of the cooling fluid flowing out of the stack so as to prevent the temperature of the stack from increasing above a predetermined temperature. Various factors could cause the temperature of the fuel stack to increase above the predetermined temperature, such as operating the stack at a high load for an extended period of time in a high ambient temperature environment.
In current fuel cell system designs, the cooling fluid temperature is typically measured at the cooling fluid outlet from the stack by a temperature sensor. If the cooling fluid is flowing, the sensor would provide an indication of stack overheating. If the cooling fluid, and thus the fuel cell stack, becomes overheated, the system may be shut down to protect the stack. However, there are potential failure modes where the system might not detect stack overheating, or detect a false overheating condition causing an unnecessary system shut down. These potential failure modes include cooling fluid pump failure, coolant fluid loss, cooling fluid flow blockage and cooling fluid outlet temperature sensor failure. If the system does not detect an overheat condition of the fuel cell stack, the stack membranes may become damaged. However, if the system falsely detects an overheating condition and shuts the system down, system reliability will be lower.